God in this verse is similar to a super-computer, a very large power source that continuously feeds on human souls to sustain itself. All human souls come from God, and remain connected to it through a metaphysical umbilical cord. A person is born with this cord, as it is transferred to them from its mother. When that person dies, that cord draws that individual to God, allowing it to subsume the soul. It then eternally feed on it like a living battery. Every human instinctually feels this connection to their god throughout their life, confirming his existence to all.
The human deity is in an arms race with multiple gods (orcs, elves, etc). Souls are power, and the more souls of a race exist, the more that particular god benefits when they go to it after death. When gods create the first batch of their race, it takes a sufficient amount of energy. However, as that race produces offspring and creates more of itself, the return of investment increases, giving the god access to more power. Therefore, gods desire their individual races to expand as far and large as possible, creating the logic behind the command "be fruitful and multiply".
In our verse, most religions have a version of hell which exists to punish those that God sees as evil, usually reflecting primal fears of that culture. Souls which fail to follow the tenets of a faith spend eternity here suffering for their sins. In this world, gods cannot afford to send souls to a separate place of torment because they depend on the collection of souls to increase their power, and discarding them would be a waste. However, as souls are very valuable, there would be no reason for people to follow the rules of faith and remain "in good standing" with their god, as everyone is going to have the same fate of being absorbed into their deity.
How can this religion rein in humans without the promise of an afterlife?